Basically, if the value is NULL, comparing it with any other value will always give you false.

cVal = 'test'
cVal = null
=messagebox(iif(cVal = 'test','Test','Not test'))
=messagebox(iif(cVal <> 'test','Not test','Test'))

So, you should always check for IsNull first and then compare with other value.


>That was my expectation as well. I am surprised (pleasantly) that it doesn't produce an error and it appears to give accurate results, but I can't find any documentation on it. I'm sure it is there somewhere in help. I found out in testing that with sql, if you don't convert it using isnull(), it can give erroneous results. For instance when the value of a bit field allows nulls:
>
>(ISNULL(myfield,0)!=1)
>
>instead of:
>
>myfield!=1
>
>but in VFP9 it doesn't appear to be necessary to convert the null value before checking it.
>
>
>>Looks like it works without it too for TRUE case, though I would assume to give an error instead.
>>

>>ll=.t.
>>ll=null
>>=messagebox(iif(ll,"True","False"))
>>
>>ll=.f.
>>ll=null
>>=messagebox(iif(not ll,"False","True"))
>>

>>
>>In other words, I would stay with NVL to give exact result.
>>
>>
>>>Is there anytime that a field which allows nulls would generate an error on the code:
>>>
>>>

>>>IF myfield.myvalue  && is the value .t.?
>>>

>>>
>>>The field value can be .T., .F., or null
>>>
>>>It is never generating an error, but I have always had the code:
>>>
>>>

>>>IF (NVL(myfield.myvalue,.F.))
>>>

>>>
>>>and am wondering if the NVL() is guaranteed to not be necesssary? I have tested the speed difference
>>>and even with over a million records the NVL() speed difference appears negligible. At onetime the
>>>NVL() was added to all fields (all types char, date, log, etc) as a safety check more or less.
>>>
>>>TIA,
>>>Tracy